1. Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines and, more particularly, to a novel spark plug for such engines which emits a low level of radio frequency electromagnetic radiation.
2. Background Art
Spark plugs are well known devices for creating a source of ignition for the combustion of fuel in internal combustion engines and may be, for example, of the construction described in U.S. Pat. No. 4,796,944, Issued Jan. 8, 1989, to Stimson. Conventional such spark plugs produce a relatively high level of radio frequency interference (RFI). In the past, such RFI was primarily of concern to the extent that it caused interference with radio broadcasts received in the vehicles in which the internal combustion engines were installed and also in nearby vehicles. Recently, however, with the advent of computer controlled ignition, carburetion, valve timing, and other functions, there is concern that RFI may cause problems with the computers controlling such functions.
The source of internal combustion engine ignition RFI is primarily due to the spark breakdown that takes place at the spark plug gaps before arcing begins. The RFI is generated by the large di/dt caused by discharging a capacitor (that has been charged to a high voltage) through a low impedance circuit. The capacitance is primarily in the structure of the spark plug itself and can range equivalently from 8.5-12 pf. The breakdown voltage is highly variable depending upon engine torque and ranges ordinarily from 8-24 KV. The initial spark breakdown current can range from 0.5-3.0 KA and the duration is on the order of 10.sup.-10 seconds. The stored energy that is discharged is 1/2CE.sup.2 .congruent.1 mj and the di/dt.congruent.10.sup.7 A/microsecond. This breakdown is followed by an arc discharge that typically is at 800 volts, 40 mA, may last 1.5 milliseconds, and may typically have 40-50 mj of energy and creates comparatively little RFI. Though the energy dissipated during arcing is 40 times that of the initial discharge, its rate of delivery is in the order of 10.sup.12 times slower. During the arcing phase, the metal conducts the heat away from its surface and the rate of surface sublimation is, therefore, less than during the breakdown phase. The RFI spectrum power associated with the spark breakdown is large, is conducted away to the outside world through the spark plug, and is principally radiated by the external high tension circuit.
Several major industry moves have reduced the radiated RFI. The first was to incorporate a series resister in the high tension circuitry which was followed by incorporating a resistor in the spark plug itself. Resistive high tension wire was also used. Inductors incorporated in the spark plug or the high tension wiring were also used with less than satisfactory overall economic results. All of these moves did reduce RFI to a greater or lesser extent. The continuing concern has been about the degree of reduction, the cost, and the associated side issues such as concern with making the ignition system more sensitive to spark plug fouling and the fact that a good portion of the ignition energy was lost in the resistors. Due to the latter factor, the ignition system power had to be increased by a factor of approximately two.
The resistive techniques gained the broadest application: however, they require both the resistor spark plug and the resistor wire to meet government and industry requirements and further future reduction of the RFI using these techniques as now applied would appear marginal. Increasing the circuit resistance still further increases the power that the ignition system requires in overcoming the associated losses. If the resistance of the spark plug is 5 K.OMEGA. and the high tension circuit is 10 K.OMEGA. and 40 mj is desired in the ignition spark gap, there would be approximately I.sup.2 Rt=(0.04).sup.2 (20.times.10.sup.3)(0.0015)=48 mj lost in the circuit. The ignition system must produce approximately 88 mj to deliver 40 mj in the spark gap. In addition to this, as the total circuit resistance increases, the ignition system becomes increasingly sensitive to spark plug fouling. The first reason that for this is that the fouling material can be carbon and/or moisture that can create a shunt resistive path which has two effects: (a) it drives down and delays the development of the ignition voltage at the spark gap, hence a higher voltage ignition system is required (hence higher energy) to overcome these effects and (b) more ignition system energy is required to overcome the losses incurred as the spark gap voltage is building to the point of breakdown. The second reason why more ignition energy is required is that if the spark gap is filled with fluid, a spark cannot take place until the fluid is vaporized by the electrical energy applied to it. This takes time and, therefore, if the circuit has a long length of resistance, the applied energy may be dissipated by external circuit resistance prior to the development of a spark.
In addition to the spark plug RFI and fouling problems, the spark plug electrode erosion rate is of great importance and is accelerated by the high rate of energy transfer during spark breakdown.
Accordingly, it is a principal object of the present invention to provide a spark plug which emits less radio frequency electromagnetic radiation.
A further object of the invention is to provide such a spark plug which can reduce the required energy of an ignition system.
An additional object of the invention is to provide such a spark plug which may be manufactured by conventional techniques.
Another object of the invention is to provide such a spark plug which can provide reduced fouling and erosion.
Other objects of the present invention, as well as particular features and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.